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Weathering and SoilsChapter 5
Earths surface processesFirst a definition: Weathering Physical breakdown and chemical alteration of rock at Earths surface
Source: Tom Bean/DRK Photo
Mechanical Weathering
Mechanical weathering breaking of rocks into smaller piecesFour types of mechanical weathering1. Frost wedging freezing and thawing of water in cracks disintegrates rocks
Frost Wedging
Evidence of Frost Wedging in Wheeler Park, Nevada
Source: Tom Bean/DRK Photo
Rockfall caused by frost wedging
Salt Wedging
Mechanical Weathering (continued)
2. Mechanical Exfoliation AKA Unloading exfoliation of igneous rocks at Earths surface due to erosion of overburden and reduction in pressure - Like peeling layers off an onion3. Thermal expansion and Contraction alternate expansion and contraction due to heating and cooling - important in deserts4. Biological activity disintegration resulting from plants and animals - root wedging, animal burrows
Mechanical Exfoliation of a Pluton
AKA Unloading
Mechanical Exfoliation in Yosemite National Park
Source: Phil Degginger/Earth Scenes
Mechanical Exfoliationof granite
AKA Unloading
Thermal Expansion and Contraction
Source: Tom Bean
Tree roots growing in rock fractures,plus animal burrows, expose deep rocks to water
Source: Runk/Schoenberger/Grant Heilman
Increase in surface area by mechanical weathering
Differential weathering controlled by jointing patternsDefinitions:Joints and Faults
Deep rock exposed to water
Joint-controlled weathering in igneous rocks
Joints in sandstone - Canyonlands Natl Park, Utah
Chemical WeatheringBreaks down rock and mineralsImportant agent in chemical weathering is water Water dissolves and transports ions and molecules The dissolved ions later bond & form the cements in sedimentary rocks
Source: Ramesh Venkatakrishnan
Chemical WeatheringMajor processes of chemical weathering
DissolutionBy carbonic acid, CO2 in water CO2 + H2O => H2CO3 H2CO3 => H+ + HCO3-Soluble ions contained in underground waterOxidationChemical reaction where compound loses electronsImportant in breaking down mafic mineralsRust colored weathering of Basalt Hydrolysis H+ or OH- ionsReaction of any substance with waterWaters ions replace different ions in mineralFeldspars, most abundant crust minerals, become fine clay particles. Easily transported.
Dissolution Weathered and Unweathered Limestone Boulders
Source: Ramesh Venkatakrishnan
Hydrolysis Feldspar to Clay
Mechanical fracture due to chemical weatheringFeldspars become
Angular Boulder Decomposes and Rounds
Angular Boulder Decomposes and Rounds
Source: Paul McKelvey/Tony Stone Images
HydrolysisCleopatras Needle, (Egypt)
Source: New York Public Library, Locan History and Genealogy DivisionGranite in aDry Climate
HydrolysisCleopatras Needle, (Central Park, NYC)
Source: Runk/Schoenberger/Grant Heilman
Granite in aWet Climate
Oxidation of BasaltRust (Iron Oxide) forms
Calcite (Limestone) No solids Ca ++ CO3--
WeatheringRates of weathering
Mechanical weathering aids chemical weathering by increasing surface areaOthers factors affecting weathering
Rock characteristics e.g. mineralsMarble and limestone easily dissolve in weak acidic solutions -Dissolution
Chemical weatheringby dissolutionLimestoneWeathering-resistant sandstone yields little soilSoilSoilSoilFeldspar-richgraniteIron-richbasaltChemicalweatheringby oxidationChemicalweatheringby hydrolysisFactors related to bedrock composition
Stability at SurfaceOthers factors affecting weathering
Rock characteristics continuedDiscontinuous series silicate minerals weather in the reverse order of their order of crystallization (Bowens Reactions)Hi T-P Olivine easiest to weather Lo T-P Quartz hardest, crystallizes closer to surface conditions
Bowens Reaction Series and Weathering
Our food grows in soils. Also very important in recognizing past climatesSoils
Soil
Soil - combination of mineral and organic matter, water, and air
It is that portion of the regolith (weathered rock and mineral) that supports the growth of plants
Components in soil that support plant growth
SoilFactors controlling soil formation
Parent materialparent material is the underlying bedrock - composition affects soil types
SoilFactors controlling soil formation
TimeSoils get better developed (Thicker, with greater differences between layers) with more timeClimateBiggest control on soil formationKey factors are temperature and precipitation
SoilFactors controlling soil formation
Plants and animalsOrganisms influence soil propertiesAlso furnish organic matter to the soil (especially plants)SlopeSteep slopes have poorly developed soils (due to faster erosion and downslope transportFlatter terrain accumulates soil faster
Variations in soil development due to topographyNote location of agriculture
Soil ProfileThe soil profile
Soil forming processes operate from the surface downwardVertical differences are called horizons zones or layers of soil
An idealized soil profilewith horizons
SoilThe soil profile
O horizon organic matterA horizon organic and mineral matterHigh biological activity (animals live here)Together the O and A horizons make up topsoilE horizon little organic matterZone of leachingB horizon zone of accumulationC horizon partly altered parent materialMnemonic: Only Active Educators Become Champions
Remember the different horizonsO
A
E
B
COrganicAnimal Activity
Exited
Back
Crushed RockSoluble MineralsSoluble Minerals
Vegetation and Soil Development
1. Plants use Hydrolysis to get nutrient metals2. Humic acids and metals returned at death. Acids remove metals from E-layer
Animal Activities in A horizonSource: Runk/Schoenberger/Grant Heilman
Eluviation & Illuviation
Illuviation and EluviationLeaching and Precipitation of Iron
Source: Jens/Gutzmer/Rand Afrikaans University/GeologyIron Leached OutPure IronIron Oxide
Soil types
The characteristics of each soil type primarily depend on the prevailing climatic conditionsThree very generic soil types
Pedalfer Moist All YearAccumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizonBest developed under forest landscapesPedocal - High EvaporationWhite calcium carbonate (caliche) in B horizonAssociated with dry grasslands and dry brush vegetationLaterite e.g. Monsoonal ClimateAlternating Hot dry and Cool Wet tropical climatesIntense chemical weatheringRed Iron oxide horizons not distinct Very thick soils but thin useful topsoil-Trees are Buttressed
Rainfall seasonalLaterite
Equator to Poles Factors
Equatorialand tropicalrain forestsSavannahsLow-latitudedeserts andsemi-desertsGrasslands(steppes)Temperate regionsand mixedboreal forestsArcticandtundraregionsEquatorAnnual precipitationIncreasing depthof weathering1800mm600mm40C30C20C10CPrecipitationTemperatureEvaporationBedrock ator very nearsurfaceSoilSoilBedrockDeeply weatheredbedrock(~40 - 50 meters deep)Temperature30 degrees Latitude
pedalferpedocallateritetropicsShortgrass Evergreen forests
Pedocal with Caliche in the B horizonO
A
E
B
COrganicActivity
Leached
Accumulation
Crushed Rock
Caliche
Source: Teaching Collection/University of Washington, Department of Geological Science
Laterite in Sarawak, Borneo
Source: Fletcher & Baylis/Photo Researchers, Inc.
Earths surface processesErosion the physical removal of material by mobile agents like water, wind, ice, or gravity
Soil ErosionSoil erosion
Recycling of Earth materialsNatural rates of soil erosion depend onSoil characteristicsClimateSlopeType of vegetation
Erosion
Source: Ramesh VenkatakrishnanHeaded for the Sea
SoilSoil erosion
In many regions the rate of soil erosion is significantly greater than the rate of soil formationFarmers now level fields with lasers to slow loss of topsoilTerraces
Soil Developed on a Lava Flow
Source: Stanley Chernicoff/Patrick SpencerEnd of Chapter 5